1. Field of Invention
This invention pertains to an image acquisition system, and more particularly to an apparatus used for the acquisition of images of a plurality of printed sheets arranged as a continuous web or a plurality of cut printed sheets fed continuously, to determine whether these printed sheets are acceptable, said apparatus including means for compensating for the speed of the sheets.
2. Description of the Prior Art
Printed sheets may be checked and validated using a device having a transport mechanism for transporting the sheets at a nominal speed past a stationary position, and scanning them with sensor means to generate electronic images thereof. The scanning may be accomplished using solid state sensors such as CCDs (Charge Coupled Devices) arranged to scan a line, self scanning silicon photodiode linear arrays, including integrating photodiode elements, or other single, linear, two-dimensional array integrating sensors, and so on. However, if the sensor integration sampling rate is held constant, all these devices are unacceptable because they produce images with geometric distortion due to the variations in the speed of the object being scanned.
To solve the geometric distortion problem, previous devices have included a synchronization device, such as a shaft encoder, on the motivating transport to synchronize the sampling rate to the motion. In this implementation, the value of the signal coming from the scanning means is not consistent because the signal is integrated over the sampling period making it proportional to the amount of light reaching the sensor and to the amount of time between samples. Therefore, variation in the movement of the sheet being scanned results in a proportional distortion of the signal.
Some previous devices have compensated for the variations in signal strength by Automatic Gain Control (AGC). This method has only a limited dynamic range and moreover the resulting image gain is affected by image content. Furthermore, the scanning means may have a significant signal to noise ratio. If the gain is adjusted too high, the resulting signal will be too noisy to produce acceptable results.
Other previous devices have compensated for the variations in sample movement by preventing the scanning means from the integrating light for a portion of the time between samples and keeping the time the scanning means is integrating constant thus obtaining a consistent signal. However, this method distorts the information because not all of the surface being scanned contributes equally to the signal. While the scanning means is being held idle, the information from the object is lost.
Yet other previous devices have corrected for speed changes by detecting the speed and adjusting a variable neutral density filter between the camera and the object. This approach requires a high degree of mechanical complexity, limited dynamic range, and a limited reaction time to rapid changes in velocity.